HPV infection is highly prevalent among young, sexually active male and female individuals. Large prospective studies showed that acquisition of HPV from male partners is common, occurring in 40-60% of subjects during a 3 year follow-up period (Koutsky et al., 1997, Ho et al., 1998, Marrazzo et al., 2000). Therefore, HPV is probably the most common sexually transmitted disease.
Papillomaviruses of the high-risk types (e. g. HPV16,18,31,33, and 45) are responsible for cervical cancer (Bosch et al., 1995, Zur Hausen, 1996). Following infection of the basal epithelial cells, the immediate HPV early genes E1, E2, E5, E6 and E7 are expressed. The E1 and E2 genes regulate viral replication. Furthermore, the E2 protein controls the expression of the E6 and E7 oncoproteins. The E6 protein of the high-risk HPV types specifically binds to p53 and targets its rapid degradation through the ubiquitin pathway. P53 is involved in initiation of apoptosis and loss of this protein result in the prevention of apoptosis (Scheffner et al., 1990). The E7 protein of high-risk types binds to pRB, which normally prevents cells from entering the cell cycle by inactivating E2F, a protein needed for cell cycle entry (Dyson et al. 1989). E7 expression results in the failure of infected cells to withdraw from the cell cycle and differentiate. Prolonged and elevated expression of the E6 and E7 oncoproteins is tightly associated with HPV-induced dysplasia and transformation into cervical carcinoma.
The protective role of the immune system in the defence against HPV-related diseases and HPV-induced cancer in humans is suggested by the fact that compared to normal controls, immunosuppressed renal transplant patients and patients infected with HIV display a 17-fold greater incidence of genital HPV infection (Ho et al., 1994, Matorras et al. 1991, Halpert et al. 1986). The diminished capacity of immunosuppressed individuals to resolve HPV infection indirectly points towards a protective role of the immune system early in infection. Evidence of protection against HPV via immunity against the early antigens E2, E6 and E7 comes from the cottontail rabbit papillomavirus model which is the major animal model for cancer-associated papillomaviruses. Vaccination with the nonstructural proteins E1 and E2 induces the regression of virus-induced papillomas whereas viral tumor growth is suppressed. Furthermore, rabbits vaccinated with the combination of the E1, E2, E6, and E7 genes were completely protected against viral challenge (Han et al. 1999, Selvakumar et al.1995). These data indicate that immunity against E2, E6, and E7 can be effective as immunoprophylaxis of papillomavirus infection as well as therapeutically for HPVinduced lesions and cancer.
Considerable interest exists in the identification of epitopes involved in the immune response to HPV16, given the possibility to incorporate these as subunits into a vaccine or to use these epitopes to monitor vaccine induced immunity in vivo. Since most epithelial cells express MHC class I but not class II, the attention has so far been focused on the induction of tumoricidal HPV-specific CD8+ cytotoxic T lymphocytes (Melief et al., 2000; Ressing et al., 1995; Ressing et al., 2000; Ressing et al., 1996). HPVspecific CD8+ T-cell reactivity has been found in the peripheral blood of patients diagnosed with cervical intraepithelial neoplasia grade III (CIN III) lesions or cervical carcinoma (Nimako et al., 1997; Ressing et al., 1996) and in tumor-infiltrating T-cell populations isolated from patients with cervical cancer (Evans et al., 1997). Tumorspecific CD4+ T helper (“Th”) immunity is now also considered pivotal for the efficient eradication of solid tumors, despite the fact that most of these tumors do not express MHC class II (reviewed in Melief et al., 2000; Pardoll and Topalian, 1998; Toes et al., 1999). Recent evidence indicates that CD4+ tumor specific T-cells are required not only for optimal induction of CD8+ tumor specific CTL but also for optimal exertion of local effector cell function by these CTL (Ossendorp et al., 1998, Toes et al., 1999). For induction of MHC class I restricted tumor-specific immunity, cross-presentation of antigens that have been captured by professional antigen presenting cells appears to play a dominant role. For proper induction of tumor-specific CTL by cross-priming tumor-specific CD4+ T cell help is required (Toes et al., 1999, Schoenberger et al., 1998).
A positive role for HPV-specific Th-immunity was suggested by the predomination of CD4+ T-cells in regressing genital warts (Coleman et al., 1994) as well as by the detection of delayed-type hypersensitivity responses to HPV16 E7 in the majority of subjects with spontaneous regressing CIN lesions (Hopfl et al., 2000). Furthermore, HPV16-specific CD4+ T-cells have been detected in the blood of patients with persistent HPV infections, high-grade CIN lesions or cervical cancer (de Gruijl et al., 1998). For the less prevalent oncogenic types HPV59 and HPV68, HLA-DR4 restricted Th-cells were isolated from the T-cells that infiltrated a cervical cancer lesion (Hohn et al., 1999). In contrast, despite the presence of HPV16 in the majority of progressive CIN lesions and cervical cancers, no in depth information is available concerning HLArestriction and epitope specificity of HPV16-specific T-helper responses. For a clinically relevant approach of immunizing subjects, for instance against (Myco) bacterially and/or virally infected cells or tumor cells, and HPV in particular, it is preferred that both specific T-helper cells and CTL are induced. We have already shown that immunization with minimal CTL epitopes results in protection against tumors in some models (Feltkamp et al. 1993, Kast et al. 1991) whereas, in other models, it can lead to tolerance or functional deletion of virus-and tumor-specific CTL that when otherwise induced are protective (Toes et al. 1996ab). The occurrence of tolerance or functional deletion decreases the effects of vaccination significantly. However, until the present invention there was no solution for this phenomenon. Epitopes involved with this effect were therefore not suitable for immunization purposes. Processing of exogenous antigens for presentation by MHC class I molecules by cross-priming as well as by other mechanisms is now widely recognized second pathway of processing for presentation by MHC class I, next to the well known endogenous route (Jondal et al. 1996, Reimann et al. 1997). The normal outcome of antigen processing via this pathway is CTL tolerance, unless APC activation by CD4+ T-cells takes place (Kurts et al., 1997).
Furthermore, in several studies with murine virus infections, a positive correlation was detected between the frequency of CTL precursors and protective immunity (Sedlik et al. 2000, Fu et al., 1999). For an optimal induction of CTL, presentation of CTL epitopes preferably takes place at the surface of professional antigen presenting cells (APC's) such as dendritic cells (Mellman e al. 1998, Rodriguez et al. 1999). Whereas minimal CTL-and Th-epitopes can, without the need of processing by professional antigen presenting cells, be presented to T-cells, proteins need to be taken up and processed for an optimal presentation of CTL and Th-epitopes in MHC class I and MHC class II, respectively, can occur (Manca et al. 1994).